Fluid filters have limited useful lifetime after which the filter medium becomes saturated with filtered out impurities or contaminants and ceases to be effective in their removal from the fluid. Means for indicating when a filter reaches the end of its useful life are important for many applications. A variety of indicator means for that purpose are available.
Most of the electronic and the mechanical indicator systems employ volume related mechanisms, i.e. totalize the fluid volume which is passed through the filter treatment media. Such indicator systems are exemplified by U.S. Pat. Nos. 5,236,578 and 5,679,243 (electronic indicator systems) and by U.S. Pat. Nos. 5,527,451; 5,536,394 and 5,882,507 (mechanical indicator systems). An alternative approach for measuring the actual utilization of the active components in the filter is disclosed in U.S. Pat. No. 5,716,912 which describes the use of a color indicator bonded to an ion exchange resin within the filter cartridge, where said indicator changes color when the ion exchange capacity of the filter becomes exhausted.
An alternative approach for determining the end of useful life of the filter takes into account the time elapsed since the activation of the filter. This approach is utilized by many of the chemical and diffusion indicator systems. Examples of such indicators include: U.S. Pat. No. 3,520,124 in which two reagent-bearing porous matrices are brought into contact, resulting in a time-dependent color changing reaction; U.S. Pat. No. 4,028,876 in which two reagents slowly mix through a porous medium to produce a color change; U.S. Pat. No. 5,667,303 in which a viscoelastic material slowly migrates into a porous medium in order to provide a visually observable indication; WO 0200552 in which water migrates into a color change mechanism and causes first and second reagents to react producing a color change.